Optical scanning apparatus and image forming device

ABSTRACT

An optical scanning apparatus includes a light source, an optical deflector having a rotary polygon mirror to deflect a light beam from the light source, a scanning optical system configured to focus the light beam deflected by the optical deflector on a target surface, a sync detecting sensor configured to determine a write start timing on the target surface, and a processing unit configured to correct detection data of the sync detecting sensor based on a measured value of a time needed for one revolution of the rotary polygon mirror.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 13/718,514, filed Dec. 18, 2012, which claims the benefit ofpriority from Japanese Patent Application Nos. 2012-265706, filed Dec.4, 2012, 2012-023572, filed Feb. 7, 2012, and 2011-277161, filed Dec.19, 2011; the entire contents of each of the above are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to an optical scanning apparatus whichscans a target surface by light, and an image forming device includingthe optical scanning apparatus provided therein.

2. Description of the Related Art

Image forming devices, such as digital multi-functional peripherals orlaser printers, are provided with an optical scanning apparatus. In theoptical scanning apparatus, a light beam emitted from a light source isdeflected by an optical deflector having a rotary polygon mirror, sothat a photoconductor drum surface is scanned by the deflected lightbeam.

Generally, an image forming device including plural photoconductor drumsuses an optical scanning apparatus having two scanning lenses arrangedin two opposed positions that confront opposite sides of the opticaldeflector respectively. In the following, the optical scanning apparatusof this type will be called an opposed scanning type optical scanningapparatus.

The optical scanning apparatus may include a sync detecting sensorarranged at a predetermined position where a light beam prior to a writestart time is received, in order to equalize write start positions forplural scanning lines in a main scanning direction on the photoconductordrum surface. In the following, an output signal of the sync detectingsensor will be called a sync detection signal.

In the opposed scanning type optical scanning apparatus, a single syncdetecting sensor may be arranged at a position confronting one of thephotoconductor drums. In this case, a false sync signal (or pseudo syncsignal) for the other photoconductor drums is generated based on thesync detection signal output from the sync detecting sensor confrontingthe one of the photoconductor drums.

Mirror surfaces of the rotary polygon mirror in the above-mentionedoptical deflector are formed by cutting. However, at a result of anormal cutting process, the angle between two adjacent ones of themirror surfaces may not be uniform. In such a case, when thephotoconductor drum surface is scanned by the light beam deflected on amirror surface different from the mirror surface from which the syncdetection signal is obtained, if a write start timing of an image isdetermined from a false sync signal generated based on the syncdetection signal, the write start position of the image will deviatefrom the correct position. If the precision of cutting is improved, theirregularity of the angle may be reduced. However, the process cost willincrease conversely.

For example, Japanese Patent No. 4,393,133 discloses an image formingdevice which is arranged to include a detection unit arranged at aposition corresponding to a first light emitting device to detect alaser beam from the first light emitting device scanned by a rotarypolygon mirror. A signal generating unit generates a horizontal syncsignal for determining the timing to form an electrostatic latent imagein a main scanning direction on an image support object with the laserbeam from the first light emitting device according to a result of thedetection by the detection unit. A measuring unit measures an intervalof the detection times that the laser beams of the first light emittingdevice scanned by the faces of the polygon mirror are detected by thedetection unit sequentially. A timing determination unit determines thetiming to form the electrostatic latent image on the image supportobject by a laser beam of a second light emitting device scanned by aface of the polygon mirror different from the face by which the laserbeam of the first light emitting device is scanned, based on thedetection of the laser beam of the first light emitting device by thedetection unit when generating the horizontal sync signal, and theinterval measured by the measuring part, without detecting the laserbeam of the second light emitting device to be scanned by the polygonmirror.

Japanese Laid-Open Patent Publication No. 2003-185952 discloses anoptical scanning apparatus in which two or more of plural scanning unitsshare a polygonal deflector. The two or more scanning units use thelight beams deflected by different deflection surfaces of the polygonaldeflector. A single write start position detecting unit is arranged todetect the light beams from the different deflection surfaces of thepolygonal deflector. The write start timing for the scanning units tothe target surface is determined by using an output signal of this writestart position detecting part.

Moreover, Japanese Laid-Open Patent Publication No. 2004-102276,Japanese Laid-Open Patent Publication No. 2006-305780, Japanese PatentNo. 3,773,884, and Japanese Laid-Open Patent Publication No. 2011-011504disclose the related technology similar to that of the above-mentionedrelated art documents.

However, in these years, the requirements for increasingly high imagequality of image forming devices are present, and there has been aproblem that the image forming device disclosed in Japanese Patent No.4,393,133 and the image forming device using the optical scanningapparatus disclosed in Japanese Laid-Open Patent Publication No.2003-185952 have difficulty in satisfying the requirements.

The inventors of the present application have examined the image qualityof an image forming device including an optical scanning apparatus inwhich a false sync signal is generated to determine a write starttiming, and have discovered that the image quality is affected by therotation irregularity of the rotary polygon mirror.

Moreover, as known conventionally, the error (the face-by-face error)for the deflection reflective surfaces of the polygon mirror is also aproblem that affects the image quality.

The main factor of the face-by-face error is the variations in thedistance from the rotation axis of the polygon mirror to the deflectionreflective surfaces of the optical deflector (or eccentricity of thepolygon mirror and profile irregularities of the faces of the polygonmirror).

One of the causes of the irregularity of the write end position is thatthe scanning irregularity is produced when two or more light sources areused. The main factor of this scanning irregularity is that there is adifference in the oscillation wavelength of the light sources, and thescanning speed is varied according to the chromatic aberration of thescanning optical system.

SUMMARY OF THE INVENTION

Accordingly, in one aspect, the present disclosure provides an opticalscanning apparatus including: a light source; an optical deflectorhaving a rotary polygon mirror to deflect a light beam from the lightsource; a scanning optical system arranged to focus the light beamdeflected by the optical deflector on a target surface; a sync detectingsensor arranged to determine a write start timing on the target surface;and a processing unit configured to correct detection data of the syncdetecting sensor based on a measured value of a time needed for onerevolution of the rotary polygon mirror.

Other objects, features and advantages of the present disclosure willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the composition of a color printer of anembodiment of the present disclosure.

FIG. 2 is a diagram showing the composition of an optical scanningapparatus shown in FIG. 1.

FIG. 3 is a diagram for explaining the faces of a polygon mirror.

FIG. 4 is a timing chart for explaining a write start timing in aphotoconductor drum.

FIG. 5 is a timing chart for explaining a conventional example 1 of afalse sync signal.

FIG. 6 is a timing chart for explaining a conventional example 2 of afalse sync signal.

FIG. 7 is a timing chart for explaining a conventional example 3 of afalse sync signal.

FIG. 8 is a timing chart for explaining a method of obtaining a timedifference between the deflection reflective surfaces in FIG. 7.

FIG. 9 is a diagram for explaining a time T1.

FIG. 10 is a diagram for explaining a time T2.

FIG. 11 is a diagram for explaining measurement data of time T1 and timeT2.

FIG. 12 is a diagram for explaining the computation result of ΔT2 andΔT2 a.

FIG. 13A and FIG. 13B are diagrams for explaining a false sync signalwhen taking the rotation irregularity into consideration.

FIG. 14 is a flowchart for explaining a method of correcting acoefficient k.

FIG. 15 is a diagram for explaining the method of correcting thecoefficient k.

FIG. 16 is a diagram for explaining an example of line patterns.

FIG. 17 is a diagram for explaining an amount of deviation ΔL of theline patterns.

FIG. 18 is a diagram for explaining correction of the rotationirregularity by a second method.

FIG. 19 is a diagram for explaining an optical scanning apparatus inFIG. 1.

FIG. 20 is a diagram for explaining a case where a light source and async detecting sensor are mounted on the same substrate.

FIG. 21 is a diagram for explaining a case where a light source and async detecting sensor are mounted on different substrates.

FIG. 22A is a timing chart for explaining the case of FIG. 21.

FIG. 22B is a timing chart for explaining the case of FIG. 20.

FIG. 23 is a timing chart for explaining a time from a sync detectioninstant to a write start instant when taking the rotation irregularityinto consideration.

FIG. 24 is a diagram showing the composition of the image forming deviceof the first embodiment.

FIG. 25 is a block diagram showing the composition of a pixel clockgenerating part-1.

FIG. 26 is a timing chart for explaining operation of the pixel clockgenerating part-1.

FIG. 27 is a block diagram showing the composition of a false signalgenerating part.

FIG. 28 is a diagram for explaining an error for each of the faces ofthe polygon mirror.

FIG. 29 is a block diagram showing the composition of a pixel clockgenerating part 2.

FIG. 30 is a diagram showing the composition of the image forming deviceof the second embodiment.

FIG. 31 is a timing chart for explaining operation of the false syncsignal generating part in the second embodiment.

FIG. 32 is a diagram showing the composition of the image forming deviceof the third embodiment.

FIG. 33 is a block diagram showing the composition of a pixel clockgenerating part 4.

FIG. 34 is a diagram for explaining the relationship between a pixelclock and image data.

FIG. 35 is a diagram showing the composition of the image forming deviceof the fourth embodiment.

FIG. 36 is a timing chart for explaining operation of the false syncsignal generating part in the fourth embodiment.

FIG. 37 is a timing chart in which a first sync signal and a second syncsignal are separated from a sync signal.

FIG. 38 is a diagram showing the composition of the image forming deviceof the fifth embodiment.

FIG. 39 is a diagram showing the composition of the image forming deviceof the sixth embodiment.

FIG. 40 is a timing chart for explaining operation of the false syncsignal generating part in the sixth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will be given of embodiments of the present disclosurewith reference to the accompanying drawings.

FIG. 1 shows the outline composition of a color printer 2000 of anembodiment of the present disclosure. As shown in FIG. 1, the colorprinter 2000 is a tandem-type multicolor printer which forms a fullcolor image by overlapping images of four colors (black, cyan, magenta,yellow). The color printer 2000 includes two optical scanningapparatuses (2010A, 2010B), four photoconductor drums (2030 a, 2030 b,2030 c, 2030 d), four cleaning units (2031 a, 2031 b, 2031 c, 2031 d),four charging units (2032 a, 2032 b, 2032 c, 2032 d), four developingrollers (2033 a, 2033 b, 2033 c, 2033 d), a transfer belt 2040, atransfer roller 2042, a fixing roller 2050, a feed roller 2054, adelivery roller 2058, a sheet feed tray 2060, a sheet output tray 2070,a communication control device 2080, and a printer control device 2090that controls the component parts of the color printer.

The communication control device 2080 controls the bidirectionalcommunications between the color printer 2000 and a host device (forexample, a personal computer) through a network.

The printer control device 2090 includes a CPU, a ROM which storesprograms executable by the CPU and various data used when executing theprograms, a RAM which provides a working area for the CPU, and an AD(analog-to-digital) converter which converts analog data into digitaldata. The printer control device 2090 controls the component parts ofthe color printer 2000 according to the instructions from the hostdevice.

In the color printer 2000, the photoconductor drum 2030 a, the chargingunit 2032 a, the developing roller 2033 a, and the cleaning unit 2031 aare used as a group and constitute a black image formation station(which will be called “K station”) that forms an image of black. Thephotoconductor drum 2030 b, the charging unit 2032 b, the developingroller 2033 b, and the cleaning unit 2031 b are used as a group andconstitute a cyan image formation station (which will be called “Cstation”) that forms an image of cyan. The photoconductor drum 2030 c,the charging unit 2032 c, the developing roller 2033 c, and the cleaningunit 2031 c are used as a group and constitute a magenta image formationstation (which will be called “M station”) that forms an image ofmagenta. The photoconductor drum 2030 d, the charging unit 2032 d, thedeveloping unit 2033 d, and the cleaning unit 2031 d are used as a groupand constitute a yellow image formation station (which will be called “Ystation”) that forms an image of yellow.

On the surface of each of the photoconductor drums, a photosensitivelayer is formed. Namely, the surface of each photoconductor drum is atarget surface to be scanned, and a latent image is formed on the targetsurface. Each of the photoconductor drums is rotated around itshorizontal rotation axis in a direction indicated by the arrow in FIG. 1by a non-illustrated rotating mechanism.

Each of the charging units electrically charges the surface of thecorresponding one of the photoconductor drums in a uniform mannerrespectively.

The optical scanning apparatus 2010A scans the charged surface of eachof the photoconductor drum 2030 a and the photoconductor drum 2030 b bythe light which is modulated for each color based on the correspondingone of the image information of black and the image information of cyanreceived from the printer control device 2090 respectively. Thereby, thelatent images corresponding to the image information received are formedon the surfaces of these photoconductor drums respectively. The latentimages formed are moved in the direction toward the corresponding one ofthe developing rollers in accordance with the rotation of thecorresponding photoconductor drum.

The optical scanning apparatus 2010B scans the charged surface of eachof the photoconductor drum 2030 c and the photoconductor drum 2030 d bythe light which is modulated for each color based on the correspondingone of the image information of magenta and the image information ofyellow received from the printer control device 2090 respectively.Thereby, the latent images corresponding to the image informationreceived are formed on the surfaces of these photoconductor drumsrespectively. The latent images formed are moved in the direction towardthe corresponding one of the developing rollers in accordance with therotation of the corresponding photoconductor drum.

The details of each of the optical scanning apparatuses will bedescribed later.

In the following, a scanning area of each photoconductor drum in whichthe image information is written will be called “effective scanningarea”, “image formation area”, or “effective image area”.

Toner from a corresponding toner cartridge (not shown) is supplied tothe surface of each of the developing rollers uniformly by the rotationof the developing roller. When the toner on the surface of eachdeveloping roller touches the surface of the correspondingphotoconductor drum, the toner adheres to only the portion of thephotoconductor drum surface which has been irradiated with the lightbeam. Namely, each developing roller serves to visualize the latentimage on the surface of the corresponding photoconductor drum into atoner image by applying the toner thereto. This toner image is moved inthe direction toward the transfer belt 2040 in accordance with therotation of the photoconductor drum.

The respective toner images of yellow, magenta, cyan, and black aresequentially transferred onto the transfer belt 2040 in predeterminedtiming, so that these toner images are overlapped to form a full colorimage.

A number of recording sheets is stored in the sheet feed tray 2060. Thefeed roller 2054 is disposed in the vicinity of the sheet feed tray2060. The feed roller 2054 picks out a single recording sheet from therecording sheets in the sheet feed tray 2060 at a time. This recordingsheet is delivered to a nip between the transfer belt 2040 and thetransfer roller 2042 in predetermined timing. Thereby, the color imageon the transfer belt 2040 is transferred to the recording sheet. Therecording sheet to which the color image is transferred is delivered tothe fixing roller 2050.

In the fixing roller 2050, heat and pressure are applied to therecording sheet so that the toner is fixed to the recording sheet. Therecording sheet to which the toner is fixed is delivered to the sheetoutput tray 2070 through the delivery roller 2058. In this manner, therecording sheets are sequentially accumulated on the sheet output tray2070.

Each of the cleaning units removes the remaining toner (residual toner)from the surface of the corresponding photoconductor drum. The surfaceof the photoconductor drum after the remaining toner is removed isreturned back to the position which confronts the corresponding chargingunit.

Next, the composition of the optical scanning apparatus 2010A will beexplained. As shown in FIG. 2, the optical scanning apparatus 2010Aincludes two light sources (2200 a, 2200 b), two coupling lenses (2201a, 2201 b), two aperture plates (2203 a, 2203 b), two cylindrical lenses(2204 a, 2204 b), a polygon mirror 2104A, two scanning lenses (2105 a,2105 b), two feedback mirrors (2106 a, 2106 b), a focusing lens 2112A, async detecting sensor 2113A, and a non-illustrated scanning controldevice A.

In the following, it is assumed that in an XYZ three-dimensionalrectangular coordinate system, the Y axis direction is a directionparallel to the longitudinal direction (or the rotational shaftdirection) of each photoconductor drum, and the Z axis direction is adirection parallel to the rotating shaft of the polygon mirror 2104A.

Each light source includes a semiconductor laser and a drive circuitwhich drives the semiconductor laser. The drive circuit of each lightsource is controlled by the scanning control device A.

In the following, a light beam emitted from the light source 2200 a iscalled “light beam LBa”, and a light beam emitted from the light source2200 b is called “light beam LBb”.

The coupling lens 2201 a converts the light beam LBa emitted from thelight source 2200 a into a generally parallel light beam. The couplinglens 2201 b converts the light beam LBb emitted from the light source2200 b into a generally parallel light beam.

The aperture plate 2203 a has an opening and adjusts the beam diameterof the light beam LBa from the coupling lens 2201 a. The aperture plate2203 b has an opening and adjusts the beam diameter of the light beamLBb from the coupling lens 2201 b.

The cylindrical lens 2204 a converges the light beam LBa passed throughthe opening of the aperture plate 2203 a with respect to the Z axisdirection near the deflection reflective surface of the polygon mirror2104A. The cylindrical lens 2204 b converges the light beam LBb havingpassed through the opening of the aperture plate 2203 b with respect tothe Z axis direction near the deflection reflective surface of thepolygon mirror 2104A. The optical system arranged between each lightsource and the polygon mirror 2104A is also called the optical system infront of the deflector.

The polygon mirror 2104A is a six-face mirror as a rotary polygonmirror, and each mirror surface serves as a deflection reflectivesurface. This rotary polygon mirror is uniformly rotated around therotating shaft thereof by a non-illustrated polygon motor, and each faceof the polygon mirror deflects the light from each cylindrical lens at aconstant angular velocity.

In the following, it is assumed that the rotary polygon mirror isrotated clockwise. The polygon motor is controlled based on an externalclock signal so that the rotational frequency of the rotary polygonmirror is set to 33300 rpm. Hence, the time for one revolution of therotary polygon mirror is about 1.8 ms.

The light beam LBa from the cylindrical lens 2204 a enters thedeflection reflective surface of the polygon mirror 2104A located on the−X side of the rotating shaft of the polygon mirror 2104A, and the lightbeam LBb from the cylindrical lens 2204 b enters the deflectionreflective surface of the polygon mirror 2104A located on the +X side ofthe rotating shaft.

The scanning lens 2105 a is located on the −X side of the polygon mirror2104A and arranged on the optical path of the light beam LBa deflectedby the polygon mirror 2104A.

The feedback mirror 2106 a guides the light beam LBa from the scanninglens 2105 a to the photoconductor drum 2030 a. That is, the surface ofthe photoconductor drum 2030 a is irradiated with the light beam LBa,and a light spot is formed on the surface of the photoconductor drum2030 a.

The scanning lens 2105 b is located on the +X side of the polygon mirror2104A and arranged on the optical path of the light beam LBb deflectedby the polygon mirror 2104A.

The feedback mirror 2106 b guides the light beam LBb from the scanninglens 2105 b to the photoconductor drum 2030 b. That is, the surface ofthe photoconductor drum 2030 b is irradiated with the light beam LBb,and a light spot is formed on the surface of the photoconductor drum2030 b.

The light spot on the surface of each photoconductor drum is moved inthe longitudinal direction of the photoconductor drum in accordance withthe rotation of the polygon mirror 2104A. The moving direction of thelight spot at this time is called “main scanning direction”, and thedirection of rotation of the photoconductor drum is called “sub-scanningdirection”.

The sync detecting sensor 2113A is arranged in a position whichreceives, through the focusing lens 2112A, a light beam directed to theoutside of the effective scanning area of the photoconductor drum 2030b. The sync detecting sensor 2113A outputs a sync detection signal tothe scanning control device A.

The sync detecting sensor 2113A is configured so that the sync detectionsignal is set to “low level” when the amount of received light issmaller than a predetermined value, and set to “high level” when theamount of received light is larger than the predetermined value. Thatis, when the sync detecting sensor 2113A receives a light beam, the syncdetection signal changes from “high level” to “low level”.

The scanning control device A determines a write start timing for thesurface of the photoconductor drum 2030 b based on the output of thesync detecting sensor 2113A (the sync detection signal).

FIG. 3 shows an example of the faces of the polygon mirror 2104A. Asshown in FIG. 3, the six deflection reflective surfaces of the polygonmirror 2104A are called “face-1”, “face-2”, “face-3”, “face-4”,“face-5”, and “face-6” counterclockwise.

For example, when the photoconductor drum 2030 b is first scanned by thelight reflected on the face-1, the photoconductor drum 2030 a is nextscanned by the light reflected on the face-3. Then, the photoconductordrum 2030 a is scanned by the light reflected on the face-2, and thephotoconductor drum 2030 b is scanned by the light reflected on theface-4. Further, the photoconductor drum 2030 a is scanned by the lightreflected on the face-3, and the photoconductor drum 2030 b is scannedby the light reflected on the face-5.

FIG. 4 is a timing chart for explaining a write start timing in thephotoconductor drum 2030 b. As shown in FIG. 4, if a rise of the outputof the sync detecting sensor 2113A is detected, the scanning controldevice A starts the writing to the photoconductor drum 2030 b afterprogress of a time Tk from the instant of the detection. This time Tk isan interval from the timing of the rise of the sync detection signal tothe write start timing, which is predetermined for each image formingdevice and stored in the memory of the scanning control device A.

In this embodiment, a sync detecting sensor corresponding to thephotoconductor drum 2030 a is not arranged, and a sync detection signalregarding the photoconductor drum 2030 a cannot be received. In thiscase, a method of determining the write start timing for thephotoconductor drum 2030 a is shown in FIG. 5. FIG. 5 shows a method ofgenerating a false sync signal in sync with the output of the syncdetecting sensor 2113A (conventional example 1). As shown in FIG. 5,this false sync signal is a signal which changes from “low level” to“high level” when a time Tr passes from the timing of a rise of theoutput of the sync detecting sensor 2113A. The time Tr is a time neededfor ⅙ of one revolution of the polygon mirror, which is predeterminedfor each image forming device.

Conventionally, as shown in FIG. 5, the writing to the photoconductordrum 2030 a is started after progress of the time Tk from the timing ofa rise of the false sync signal. However, a manufacturing error mayexist in the polygon mirror, and there is a problem that, if the polygonmirror has an error in the deflection reflective surfaces, the writestart positions differ.

A first method for eliminating the problem is, as shown in FIG. 6(conventional example 2), to determine a write start timing whenscanning the photoconductor drum 2030 a using the light reflected on thesame deflection reflective surface based on the output of the syncdetecting sensor 2113A. In FIG. 6, a time Tr′ indicates an intervalcorresponding to an integral multiple of the time Tr above.

A second method for eliminating the problem is, as shown in FIG. 7(conventional example 3), that time differences between the deflectionreflective surfaces (Te13, Te24, Te35, Te46, Te51, Te62) are computedbeforehand, the time Tr is corrected, and the write start timing for thephotoconductor drum 2030 a is determined by taking the time differencesinto consideration. In FIG. 7, Te13 denotes a time difference betweenthe face-1 and the face-3, Te24 denotes a time difference between theface-2 and the face-4, and Te35 denotes a time difference between theface-3 and the face-5. Te46 denotes a time difference between the face-4and the face-6, Te51 denotes a time difference between the face-5 andthe face-1, and Te62 denotes a time difference between the face-6 andthe face-2.

In order to perform the method of FIG. 7, it is necessary to compute thetime differences between the deflection reflective surfaces beforehand.First, in this method, as shown in FIG. 8, the following times aremeasured based on the output of the sync detecting sensor 2113A: a timeT12 for the face-1 (as the deflection reflective surface reflecting thelight beam) to be moved to the face-2; a time T23 for the face-2 (as thedeflection reflective surface reflecting the light beam) to be moved tothe face-3; a time T34 for the face-3 (as the deflection reflectivesurface reflecting the light beam) to be moved to the face-4; a time T45for the face-4 (as the deflection reflective surface reflecting thelight beam) to be moved to the face-5; a time T56 for the face-5 (as thedeflection reflective surface reflecting the light beam) to be moved tothe face-6; and a time T61 for the face-6 (as the deflection reflectivesurface reflecting the light beam) to be moved to the face-1.

Next, a difference of the time T12 and the time Tr is computed and thetime difference Te12 between the face-1 and the face-2 is set to thecomputed difference. A difference of the time T23 and the time Tr iscomputed and the time difference Te23 between the face-2 and the face-3is set to the computed difference. A difference of the time T34 and thetime Tr is computed and the time difference Te34 between the face-3 andthe face-4 is set to the computed difference. A difference of the timeT45 and the time Tr is computed and the time difference Te45 between theface-4 and the face-5 is set to the computed difference. A difference ofthe time T56 and the time Tr is computed and the time difference Te56between face-5 and face-6 is set to the computed difference. Adifference of the time T61 and the time Tr is computed and the timedifference Te61 between the face-6 and the face-1 is set to the computeddifference.

A time difference Te13 is determined by a sum of the time differenceTe12 and the time difference Te23. A time difference Te24 is determinedby a sum of the time difference Te23 and the time difference Te34. Atime difference Te35 is determined by a sum of the time difference Te34and the time difference Te45. A time difference Te46 is determined by asum of the time difference Te45 and the time difference Te56. A timedifference Te51 is determined by a sum of the time difference Te56 andthe time difference Te61. A time difference Te62 is determined by a sumof the time difference Te61 and the time difference Te12.

In any of the above-mentioned methods, the elapsed time from thereception of the light beam at the sync detecting sensor 2113A to thewrite start timing for the photoconductor drum 2030 a is longer than theelapsed time in the case where the sync detecting sensor correspondingto the photoconductor drum 2030 a is disposed.

The inventors of the present application have examined the image qualityof an image forming device including an optical scanning apparatus inwhich a false sync signal is generated to determine the write starttiming, and have discovered that the image quality is affected by therotation irregularity of the rotary polygon mirror. Moreover, theinventors have discovered that the larger the elapsed time from thereception of the light beam at the sensor to the write start timing, thelarger the influence by the rotation irregularity of the rotary polygonmirror.

As an example, as shown in FIG. 9, a time from the reception of thelight beam reflected on one deflection reflective surface (the face-1 inFIG. 9) at the sync detecting sensor 2113A to the reception of the lightbeam reflected again on the same deflection reflective surface is set toT1. Namely, this time T1 is a measured value of a time for onerevolution of the rotary polygon mirror.

As an example, as shown in FIG. 10, it is assumed that a virtual syncdetecting sensor corresponding to the photoconductor drum 2030 a (whichsensor is indicated as a false sync detecting sensor in FIG. 10) isarranged in the position which outputs the false sync signal as shown inFIG. 5. In this case, a time from the reception of the light beamreflected on one deflection reflective surface (the face-1 in FIG. 10)at the sync detecting sensor 2113A to the reception of the light beamreflected on the same deflection reflective surface at the false syncdetecting sensor is set to T2(<T1).

Conventionally, it was considered that both the time T1 and the time T2have a constant value. The inventors have measured the time T1 and thetime T2 experimentally. FIG. 11 shows the measurement data (1000measurement data pieces) of T1 until the cumulative rotation time of therotary polygon mirror reaches 300 ms, and the measurement data (1000measurement data pieces) of T2 measured in sync with the measurement ofT1.

As is apparent from FIG. 11, T1 and T2 were not constant and both werevaried. It can be understood that variations in T1 and T2 are due to therotation irregularity of the rotary polygon mirror.

In this example, the average of the 1000 measurement data pieces of T1was 1801.804 μs and the average of the 1000 measurement data pieces ofT2 was 1263.083 μs. Even if the cumulative rotation time is extendedbeyond 300 ms, there was no significant difference in the values ofthese averages.

In the following, “T1ave” denotes the average of plural measurement datapieces of T1, and “T2ave” denotes the average of plural measurement datapieces of T2. A difference ΔT1 of T1 and T1ave, and a difference ΔT2 ofT2 and T2ave are represented by the following formulas:

ΔT1=T1−T1ave   (1)

ΔT2=T2−T2ave   (2)

In addition, a difference ΔT2 a of T2 and T2ave when the T2ave iscorrected by considering the rotation irregularity of the rotary polygonmirror is represented by the following formula.

ΔT2a=T2−T2ave(T1−/T1ave)   (3)

FIG. 12 shows the computation results of ΔT2 and ΔT2 a described above.As shown in FIG. 12, the variations of ±0.015 μs(=±15 ns) were presentfor ΔT2, and they were reduced to the variations of ±0.004 μs(=±4 ns)which were present for ΔT2 a.

According to this experiment, it can be understood that 73%(=1−4/15) ofthe variations in T2 were due to the rotation irregularity. It wasconfirmed that the variations in T2 are greatly dependent on thevariations in T1 (rotation irregularity).

If T2 is set up to meet the condition ΔT2 a=0 (i.e., the followingequation (4) is given), the influence of the rotation irregularity in T2can be reduced.

T2−T2ave(T1/T1ave)=0   (4)

If the above equation (4) is modified, the following equation (5) isobtained.

T2=(T2ave/T1ave)T1   (5)

According to the experiments, T1ave=1801.804 μs and T2ave=1263.083 μs,the following formula (6) is obtained from the above equation (5):

T2=0.70101×T1   (6)

This shows that, if the false sync signal is changed from low level tohigh level at the timing of 0.70101 revolutions of the rotary polygonmirror after the reception of the light beam reflected on one deflectionreflective surface (e. g., the face-1 in FIG. 10) by the sync detectingsensor 2113A, the influence of the rotation irregularity can beprevented (see FIG. 13A and FIG. 13B).

However, in the image forming device, no sync detecting sensorcorresponding to the photoconductor drum 2030 a is arranged, and themeasurement data of T2ave cannot be obtained.

To eliminate the problem, the above equation (5) is rewritten into thefollowing equation (7) using a coefficient k.

T2=k×T1   (7)

FIG. 14 is a flowchart for explaining a method of determining acoefficient kin the image forming device.

As shown in FIG. 14, at step S401, a default value of the coefficient kindicating a time for one revolution of the rotary polygon mirror from atime the sync detecting sensor 2113A receives the light beam reflectedon one deflection reflective surface (for example, the face-1) to a timethe above-described false sync detecting sensor receives the light beamreflected on the deflection reflective surface is computed.

At step S403, as shown in FIG. 15, a false sync signal is generatedusing the default value of the coefficient k, and the line patterns (seeFIG. 16) which are the same as those used in the known color matchingcorrection process (see Japanese Laid-Open Patent Publication No.2011-197134) are formed. In FIG. 15, T1 ₁-T1 ₄ denote the measurementvalues of T1 obtained at the preceding cycle.

At step S405, a difference AL (see FIG. 17) in the position in the mainscanning direction between the line pattern of black and the linepattern of cyan is measured using the sensors (not shown) used in theabove-mentioned known color matching correction process.

At step S407, the average of the above-mentioned AL is converted into anamount of rotation of the polygon mirror.

At step S409, the coefficient k is corrected based on the amounts ofrotation so as to set the average of the above-mentioned ΔL to 0. Thecorrected coefficient k is used as the coefficient k of the aboveequation (7) for the image forming device.

Upon power-up, the printer control device 2090 performs an image processcontrol procedure if any of the following events take place: (1) thestop time of the photoconductor drum is over 6 hours; (2) the insidetemperature of the device has changed by 10 degrees C. or more; (3) therelative humidity of the device has changed by 50% or more; (4) thenumber of printed sheets has reached a predetermined number in theprinting job; (5) the number of times of rotation of the developingroller has reached a predetermined number; and (6) the total runningdistance of the transfer belt has reached a predetermined distance.

Hence, the printer control device 2090 may be configured to perform theabove method of determining the coefficient k during execution of theimage process control procedure. In this case, the corrected coefficientk obtained at the preceding cycle may be used, instead of the defaultvalue. Thereby, the false sync signal can be generated with a higherlevel of accuracy.

When the above-mentioned second method is used, the false sync signalcan be generated with a higher level of accuracy by taking the rotationirregularity into consideration.

As shown in FIG. 18, Tr×(T1/T1ave) is used instead of Tr. In FIG. 18, T1₁-T1 ₈ denote the measurement values of T1 obtained at the precedingcycle.

It is expected that, due to a detection error of the sync detectingsensor 2113A or the like, an unusual value is included in the measuredvalues of T1. When the measurement values of T1 obtained at thepreceding cycle do not fall within a predetermined range, themeasurement values of Ti obtained before the preceding cycle may beused. Alternatively, plural sets of the measurement values of T1 forplural cycles may be stored as history information, and the average ofsuch values may be used.

Next, the composition of the optical scanning apparatus 2010B will bedescribed. As shown in FIG. 19, this optical scanning apparatus 2010Bincludes two light sources (2200 c, 2200 d), two coupling lenses (2201c, 2201 d), two aperture plates (2203 c, 2203 d), two cylindrical lenses(2204 c, 2204 d), a polygon mirror 2104B, two scanning lenses (2105 c,2105 d), two feedback mirrors (2106 c, 2106 d), a focusing lens 2112B, async detecting sensor 2113B, and a non-illustrated scanning controldevice B.

Each light source includes a semiconductor laser and a drive circuitwhich drives the semiconductor laser. Each drive circuit is controlledby the scanning control device B.

In the following, a light beam emitted from the light source 2200 c iscalled “light beam LBc”, and a light beam emitted from the light source2200 d is called “light beam LBd”.

The coupling lens 2201 c converts the light beam LBc emitted from thelight source 2200 c into a generally parallel light beam. The couplinglens 2201 d converts the light beam LBd emitted from the light source2200 d into a generally parallel light beam.

The aperture plate 2203 c has an opening and adjusts the beam diameterof the light beam LBc from the coupling lens 2201 c. The aperture plate2203 d has an opening and adjusts the beam diameter of the light beamLBd from the coupling lens 2201 d.

The cylindrical lens 2204 c converges the light beam LBc having passedthrough the opening of aperture plate 2203 c with respect to the Z axisdirection near the deflection reflective surface of the polygon mirror2104B. The cylindrical lens 2204 d converges the light beam LBd passedthrough the opening of the aperture plate 2203 d with respect to the Zaxis direction near the deflection reflective surface of the polygonmirror 2104B. The optical system arranged between each light source andthe polygon mirror 2104B is also called the optical system in front ofthe deflector.

The polygon mirror 2104B is a six-face mirror as a rotary polygonmirror, and each mirror surface serves as a deflection reflectivesurface. This polygon mirror is uniformly rotated around the center ofthe rotating shaft of the polygon mirror 2104B parallel to the Z axisdirection, and each face of the polygon mirror deflects the light fromeach cylindrical lens at a constant angular velocity. In the following,it is assumed that the polygon mirror is rotated clockwise.

The light beam LBc from the cylindrical lens 2204 c enters thedeflection reflective surface located on the -X side of the rotatingshaft of the polygon mirror 2104B, and the light beam LBd from thecylindrical lens 2204 d enters the deflection reflective surface locatedon the +X side of the rotating shaft.

The scanning lens 2105 c is located on the −X side of the polygon mirror2104B, and arranged on the optical path of the light beam LBc deflectedby the polygon mirror 2104B.

The feedback mirror 2106 c guides the light beam LBc from the scanninglens 2105 c to the photoconductor drum 2030 c by reflection. That is,the surface of the photoconductor drum 2030 c is irradiated with thelight beam LBc, and a light spot is formed on the photoconductor drum2030 c surface.

The scanning lens 2105 d is located on the +X side of the polygon mirror2104B, and arranged on the optical path of the light beam LBd deflectedby the polygon mirror 2104B.

The feedback mirror 2106 d guides the light beam LBd from the scanninglens 2105 d to the photoconductor drum 2030 d by reflection. That is,the surface of the photoconductor drum 2030 d is irradiated with thelight beam LBd and a light spot is formed on the photoconductor drum2030 d surface.

The light spot on the surface of each photoconductor drum is moved inthe longitudinal direction of the photoconductor drum in accordance withthe rotation of the polygon mirror 2104B. The moving direction of thelight spot at this time is called main scanning direction, and thedirection of rotation of the photoconductor drum is called sub-scanningdirection.

The sync detecting sensor 2113B is arranged in a position whichreceives, through the focusing lens 2112B, a light beam directed to theoutside of the effective scanning area of the photoconductor drum 2030d. The sync detecting sensor 2113B outputs a sync detection signal tothe scanning control device B.

The scanning control device B determines a write start timing for thesurface of the photoconductor drum 2030 d based on the sync detectionsignal output from the sync detecting sensor 2113B.

Similar to the scanning control device A, the scanning control device Bgenerates a false sync signal based on a measured value of a time forone revolution of the rotary polygon mirror of the sync detection signaloutput from the sync detecting sensor 2113B, and determines the writestart timing for the surface of the photoconductor drum 2030 d.

As described above, according to the foregoing embodiment, the opticalscanning apparatus includes two light sources, two coupling lenses, twoaperture plates, two cylindrical lenses, a polygon mirror, two scanninglenses, two feedback mirrors, a focusing lens, a sync detecting sensor,and a scanning control device. The scanning control device determinesthe write start timing for the surface of the photoconductor drumcorresponding to the sync detecting sensor based on the output of thesync detecting sensor. The scanning control device determines the writestart timing for the surface of the photoconductor drum with which nosync detecting sensor is arranged, based on the output of the syncdetecting sensor and the measured value of the time for one revolutionof the rotary polygon mirror.

In this case, by taking into consideration the rotation irregularity ofthe polygon mirror, the write start timing for the surface of thephotoconductor drum with which no sync detecting sensor is arranged canbe determined. It is possible to determine the write start timing forthe photoconductor drum with which no sync detecting sensor is arrangedwith a high level of accuracy. As a result, the variations of the writestart position on the target surface can be reduced.

The color printer 2000 is provided with the optical scanning apparatus2010A and the optical scanning apparatus 2010B. As a result, the colorprinter 2000 can increase the image quality.

For the purpose of the cost reduction or miniaturization, a syncdetecting sensor may be mounted on a substrate on which the light sourceis mounted (see FIG. 20). In this case, the time from the reception of alight beam by the sync detecting sensor to the write start timing islarger than that in the case where a sync detecting sensor is mounted onanother substrate different from the substrate on which the light sourceis mounted (see FIG. 21), as shown in FIG. 22A and FIG. 22B. The timingchart of FIG. 22A illustrates the case of FIG. 21, and the timing chartof FIG. 22B illustrates the case of FIG. 20.

The case in which the sync detecting sensor is mounted on the substrateon which the light source is mounted is susceptible to the influence ofrotation irregularity, and there is a possibility that the write startposition is shifted. In this case, the time from the reception of thelight beam at the sync detecting sensor to the write start timing iscorrected by considering the rotation irregularity as in theabove-mentioned embodiment, and the variations of the write startposition can be reduced.

Specifically, as shown in FIG. 23, “t×(T1/T1ave)” is used as the timefrom the reception of the light beam at the sync detecting sensor to thewrite start timing, instead of the time “t” computed according to therelated art. In FIG. 23, T1 ₁-T1 ₈ denote the measurement values of T1obtained at the preceding cycle.

Alternatively, a surface emission-type laser array having pluralemitting parts may be used as the semiconductor laser in each lightsource in the above-mentioned embodiment.

Alternatively, the optical scanning apparatus 2010A and the opticalscanning apparatus 2010B in the above-mentioned embodiment may beunified.

In the above-mentioned embodiment, the color printer 2000 including thefour photoconductor drums has been explained as an example of the imageforming device of the present disclosure. However, the presentdisclosure is not limited to this embodiment. For example, the imageforming device of the present disclosure is applicable to a printerhaving two photoconductor drums or a printer having five photoconductordrums.

In the above-mentioned embodiment, the case where the optical scanningapparatus is used in the color printer has been explained as an exampleof the present disclosure. However, the present disclosure is notlimited to this embodiment. For example, the present disclosure isapplicable to other image forming devices, such as a copying machine, afacsimile machine, or a multi-function peripheral.

Next, a first embodiment of the present disclosure will be described.FIG. 24 is a diagram showing the composition of the image forming deviceof the first embodiment of the present disclosure.

As shown in FIG. 24, this image forming device includes two lightsources (2200 a, 2200 b), two coupling lenses (2201 a, 2201 b), twoaperture plates (2203 a, 2203 b), two cylindrical lenses (2204 a, 2204b), a polygon mirror (2104A), two scanning lenses (2205 a, 2205 b), twofeedback mirrors (2206 a, 2206 b), two photoconductors 2208 a and 2208b, a focusing lens 2112A, a sync detecting sensor 2113A, and a pixelclock generating device 120.

The pixel clock generating device 120 includes a pixel clock generatingpart-1 (111), a false sync signal generating part 113, a pixel clockgenerating part-2 (112), a pixel clock generating part-3 (114), a firstmodulation data generating part 115, a second modulation data generatingpart 118, a first laser driver 116, and a second laser driver 119.

A laser incident light beam 2207 a from the light source 2200 a entersthe polygon mirror 2104A, and in sync with the rotation of the polygonmirror 2104A, passes through the scanning lens 2205 a, so that thesurface of the photoconductor 2208 a is scanned by the light beam. Onthe other hand, a laser incident light beam 2207 b from the light source2200 b enters the polygon mirror 2104A, and in sync with the rotation ofthe polygon mirror 2104A, passes through the scanning lens 2205 b, sothat the surface of the photoconductor 2208 b is scanned by the lightbeam. Thereby, an electrostatic latent image according to the output ofthe light source 2200 a is formed on the surface of photoconductor 2208a and an electrostatic latent image according to the output of the lightsource 2200 b is formed on the surface of photoconductor 2208 b.

The sync detecting sensor 2113A is disposed at an end portion of thephotoconductor 2208 a. The laser beam reflected by the polygon mirror2104A enters the sync detecting sensor 2113A before the scanning of ascanning line by the laser beam is performed on the surface of thephotoconductor 2208 a. The sync detecting sensor 2113A detects thetiming of a start of the scanning. The timing of the start of thescanning is detected by the sync detecting sensor 2113A is supplied tothe pixel clock generating part-1 (111) and the false sync signalgenerating part 113 of the pixel clock generating device 120 as a firstsync signal that is present periodically in sync with the scanning ofthe photoconductor.

The pixel clock generating part-1 (111) generates a first pixel clockand a frequency correction value based on the first sync signal. Thefalse sync signal generating part 113 generates a false sync signalbased on the first sync signal and the first pixel clock. The pixelclock generating part-2 (112) corrects the initial frequency settingvalue by the frequency correction value and generates a second pixelclock in sync with the first sync signal. The pixel clock generatingpart-3 (114) corrects the initial frequency setting value by thefrequency correction value and generates a third pixel clock in syncwith the false sync signal.

Based on the first image data, the first modulation data generating part115 outputs first modulation data to the first laser driver 116, thefirst modulation data being synchronized with the second pixel clock.The first laser driver 116 drives the light source 2200 a according tothe first modulation data, and the light source 2200 a emits a laserbeam. Based on the second image data, the second modulation datagenerating part 118 outputs second modulation data to the second laserdriver 119, the second modulation data being synchronized with the thirdpixel clock. The second laser driver 119 drives the light source 2200 baccording to the second modulation data, and the light source 2200 bemits a laser beam.

FIG. 25 is a block diagram showing the composition of the pixel clockgenerating part-1 (111).

As shown in FIG. 25, the pixel clock generating part-1 (111) includes afirst counter 201, a moving-average computing unit 203, a filter 204, adivider 205, a delay unit 206, a register 207, a digital clockoscillator 208, a comparator 209, and an adder 210.

The first counter 201 outputs a counted value of the first pixel clockindicating the interval of the first sync signal corresponding to theinterval in which one main scanning line for the polygon mirror's oneface amount is scanned. The comparator 209 compares the counted valuewith a reference number Nref of the polygon mirror's one face amount,and supplies a difference of the counted value and the reference numberNref to the moving-average computing unit 203.

Assuming that E_(R) denotes a ratio of effective scanning period, vdenotes a photoconductor linear velocity, L denotes an effective writewidth, ρ_(m) denotes a picture element density of main scanningdirection, ρ_(s) denotes a picture element density of sub-scanningdirection, and M denotes the number of writing beams, the referencenumber Nref for the polygon mirror's one face amount is represented bythe following formula:

${Nref} = {\frac{vL}{E_{R}}\frac{\rho_{m}\rho_{s}}{25.4^{2}M}}$

For example, when the number of the faces of the polygon mirror is four,the moving-average computing unit 203 computes the moving average of thedifference values for the polygon mirror's four face amount. Thecomputed value of the moving average of the difference values for thepolygon mirror's four face amount is smoothed by the filter 204, anddivided by Nref at the divider 205, so that it is converted into theerror Δf_now per period of 15 one pixel. The delay unit 206 adds theerror Δf_now per period of one pixel to the frequency correction valueΔf as a control value. The adder 210 outputs the sum value of thefrequency correction value Δf and the initial frequency fclk_i stored inthe register 207. Then, the first pixel clock is generated by thedigital clock oscillator 208. By this feedback control, the error perperiod of one pixel is made to fall within a predetermined range. Whenthe error is within the predetermined range, the frequency correctionvalue Δf at that time is supplied to the pixel clock generating part-2(112) and the pixel clock generating part-3 (114).

The value of Nref and the initial frequency fclk_i of the register 207may be determined by the ratio E_(R) of effective scanning period, thephotoconductor linear velocity v, the effective write width L, thepicture element density ρ_(m) of main scanning direction, the pictureelement density ρ_(s) of sub-scanning direction, and the writing beamnumber M of the image forming device concerned.

FIG. 26 is a timing chart for explaining operation of the pixel clockgenerating part-1 (111).

In the example of FIG. 26, the interval Tspsp of the first sync signalfor the polygon mirror's one face received from the PD 110 is varied asthe rotational speed of the polygon mirror 100 and an error arises. iffclk_w denotes the frequency of the first control clock as a controlvalue, the period of the first pixel clock is represented by 1/fclk_w.The frequency fclk_w is controlled to make the counted value of thefirst pixel clock equal to the reference number Nref. At this time, thefollowing relationship is met.

Tspsp=Nref/fclk_w

On the other hand, if Tspsp_target denotes the target period determinedfor each model of the image forming device and fclk_i denotes theinitial frequency, the following relationship is met.

Tspsp_target=Nref/fclk_i

If Δf denotes the error of the frequency, the following condition ismet.

1/Δf=1/fclk_w−1/fclk_i

The time error At by the error of the scanning speed is

$\begin{matrix}{{\Delta t} = {{Tspsp} - {Tspsp\_ target}}} \\{= {{Nref}\left( {1 - {/{fclk\_ w}} - {1/{fclk\_ i}}} \right)}} \\{= {{Nref}/{{\Delta f}.}}}\end{matrix}$

Namely, it is possible to correct the frequency of the first pixel clockaccording to the rotational error of the polygon mirror by comparing therotational error for one revolution of the polygon mirror with thereference number Nref (when no error between the faces of the polygonmirror exists) multiplied by the number of faces of the polygon mirror.

When scanning the photoconductor using a common polygon mirror, asimilar frequency error Δf arises in the second pixel clock and thethird pixel clock. Therefore, it is possible to correct the frequency ofeach pixel clock according to the rotational error for one revolution ofthe polygon mirror by applying the frequency error Δf the second pixelclock and the third pixel clock as a correction value.

FIG. 27 is a block diagram showing the composition of the false signalgenerating part 113. As shown in FIG. 27, the false sync signalgenerating part 113 includes a splitter 401, a second counter 402, acomparator 403, and an OR circuit 404.

The first sync signal from the PD 110 is distributed to the splitter 401for the faces of the polygon mirror. For example, the first sync signalof the 0th face of the polygon mirror is distributed to the splitter401-0, and the second counter 402-0 counts the number of the first pixelclocks (or the pixel number) starting from the first sync signal of the0th face. Similarly, the first sync signal of the first face of thepolygon mirror is distributed to the splitter 401-1, 1.0 and the secondcounter 402-1 counts the number of the first pixel clocks (or the pixelnumber) starting from the first sync signal of the first face. The firstsync signal of the second face of the polygon mirror is distributed tothe splitter 401-2, and the second counter 402-2 counts the number ofthe first pixel clocks (or the pixel number) starting from the firstsync signal of the second face. The counted value after the counting iscarried out is compared with a predetermined false sync count numberNref_ps by the comparator 403. The output of the comparator 403 isasserted when the counted value=the false sync count number Nref_ps. TheOR circuit 404 takes the OR of the asserted outputs of the respectivecomparators and outputs a false sync signal indicating the OR result.

In the example of FIG. 27, the polygon mirror having the four faces, thesplitter (the splitters 0-2), the counter (the counters 0-2), and thecomparator (the comparators 0-2) are illustrated. The number of theelements necessary for each of the splitter 401, the second counter 402,and the comparator 402 is smaller than the number of faces of thepolygon mirror. The number and combination of the elements, such as thecounter, may be appropriately selected by the number of faces of thepolygon mirror used.

Next, an error of the radius of the inscribed circle of a polygon mirrorwhich may cause a deviation of a write start position on a targetsurface to be scanned will be described. FIG. 28 is a diagram forexplaining an error for each of the faces of the polygon mirror.

In FIG. 28, “a” denotes a distance from the center of a first polygonmirror to the face of the first polygon mirror, and “b” denotes adistance from the center of a second polygon mirror to the face of thesecond polygon mirror. If the incident light enters at the same positionbut the center-to-face distances of the polygon mirrors differ, thelight beam is reflected at different positions on the polygon mirrorfaces, and the write positions on the photoconductor surfaces differ.Hence, if a false sync signal is generated with respect to the polygonmirror face which is the same as the face of the polygon mirror wherethe counting is started, the above-described error does not arise.Therefore, the false sync count number Nref_ps must be determined suchthat a false sync signal is generated with respect to the polygon mirrorface which is the same as the face of the polygon mirror where thecounting is started. The false sync count number Nref_ps is stored in anon-illustrated register. In addition, there may be a case in which amirror angle error of the polygon mirror is taken into consideration.

The write start positions of the photoconductor drum 103 and thephotoconductor drum 104 may be shifted due to the errors specific to thescanning optical system, such as mounting positions of the scanninglenses 101 and 102, and manufacture errors of lens curved surfaces. Asdisclosed in Japanese Laid-Open Patent Publication No. 2004-102276,there is known a method of detecting a write start position. In thismethod, predetermined positioning marks are used as reference marks whencombining the images of the photoconductor drums 103 and 104, anddetected by positioning sensors, so that the amount of deviation iscomputed based on the detection results of the sensors.

The false sync count number Nref_ps is set up based on the computationresult of the amount of deviation so as to correct the error for eachscanning optical system.

FIG. 29 is a block diagram showing the composition of the pixel clockgenerating part-2 (112). The composition of the pixel clock generatingpart-2 (112) will be described. In this respect, the pixel clockgenerating part-3 (114) has the same composition as the pixel clockgenerating part-2 (112), and a description thereof will be omitted.

As shown in FIG. 29, the pixel clock generating part-2 (112) includes aninitial frequency setting unit 602, an adder 603, and a digital controloscillator (DCO) 604. The initial frequency setting unit 602 stores aninitial value of the frequency of the pixel clock 2. The adder 603 addsthe frequency correction value generated by the first pixel clockgenerating part to the initial frequency, so that the rotational errorof the polygon mirror is corrected. The adder 603 outputs the resultingfrequency setting value (in which the rotational error of the polygonmirror is corrected) to the digital control oscillator 604. Taking thephase synchronization to the first sync signal, the digital controloscillator (DCO) 604 generates the second pixel clock based on thefrequency set up by the adder.

In the pixel clock generating part-2 (112), an initial value differentfrom that of the first pixel clock generating part can be set up, and itis possible to perform the setting of the initial value so as to correctthe deviation of the write end position due to the error for eachscanning optical system. When the image forming device uses plural lightsources, it is advantageous that the initial value of the frequency canbe set up separately, in order to prevent the error for each scanningoptical system. There is known a method of detecting a write endposition as disclosed in Japanese Laid-Open Patent Publication No.2004-102276. In this method, the predetermined positioning marks areused as reference marks when combining the images of the photoconductordrums 103 and 104, detected by the positioning sensors, and the amountof deviation is computed based on the detection results of the sensors.The initial value of the frequency is set up based on the computationresult of the amount of deviation.

Since the write end position may be varied over time, it is preferred toset up the initial value of the frequency appropriately.

Next, a second embodiment of the present disclosure will be described.FIG. 30 is a diagram showing the composition of the image forming deviceof the second embodiment of the present disclosure.

As shown in FIG. 30, the image forming device 10 of the secondembodiment includes two light sources (LD 117, LD 127), twophotoconductor drums (103, 104), and one sync detecting sensor (PD 110).

Specifically, as shown in FIG. 30, the image forming device 10 includesa polygon mirror 100, scanning lenses 101 and 102, photoconductor drums103 and 104, incidence mirrors 105 and 106, a PD (photodetector) 110, apixel clock generating device 140, and light sources (LD 117 and LD127). The pixel clock generating device 140 includes a first pixel clockgenerating part 141, a false sync signal generating part 143, a secondpixel clock generating part 142, a third pixel clock generating part144, a first modulation data generating part 145, a second modulationdata generating part 147, a first laser driver 146, and a second laserdriver 148.

The laser incident light beam Bk from the light source 117 is reflectedby the incidence mirror 105 to enter the polygon mirror 100, and in syncwith the rotation of the polygon mirror 100, passes through the scanninglens 101, so that the surface of the photoconductor (Bk) 103 is scannedby the light beam.

On the other hand, the incident light beam Y from the light source 127is reflected by the incidence mirror 106 to enter the polygon mirror100, and in sync with the rotation of the polygon mirror 100, passesthrough the scanning lens 102, so that the surface of the lightphotoconductor (Y) 104 is scanned by the light beam. Thereby,electrostatic latent images according to the outputs of the light source117 and the light source 127 are formed on the photoconductor drum 103and the photoconductor drum 104, respectively.

The PD 110 is arranged at one end potion of the photoconductor drum 103.The laser beam reflected by the polygon mirror 100 enters the PD 110before the scanning of a scanning line by the laser beam is performed onthe surface of the photoconductor drum 103, and the timing of a start ofthe scanning is detected by the PD 110. The timing of the start of thescanning detected by the PD 110 is supplied to the pixel clockgenerating part-1 (141) of the pixel clock generating device 140, andthe false sync signal generating part 143 as a first sync signal in syncwith the scanning of the photoconductor drum.

The pixel clock generating part-1 (141) generates a first pixel clockand a frequency correction value based on the first sync signal. Thefalse sync signal generating part 143 generates a false sync signalbased on the first sync signal and the first pixel clock. The pixelclock generating part-2 (142) corrects a predetermined initial frequencysetting value by the frequency correction value, and generates a secondpixel clock in sync with the first sync signal. The pixel clockgenerating part-3 (144) corrects a second predetermined initialfrequency setting value (which is separate from the initial frequencysetting value set by the pixel clock generating part-2 (142)) by thefrequency correction value, and generates a third pixel clock in syncwith the false sync signal.

Based on the first image data, the first modulation data generating part145 outputs first modulation data in sync with the second pixel clock tothe first laser driver 146. The first laser driver 146 drives the lightsource 117 according to the first modulation data, and the light source117 emits a laser beam. Based on the second image data, the secondmodulation data generating part 147 outputs second modulation data insync with the third pixel clock to the second laser driver 148. Thesecond laser driver 148 drives the light source 127 according to thesecond modulation data, and the light source 127 emits a laser beam.

In the second embodiment, the polygon mirror having the four faces isused, and the two of the faces of the polygon mirror are used to performthe scanning of the two photoconductors as shown in FIG. 30. However,the PD 110 is arranged only at one of the two photoconductors, and thescanning of the photoconductor with which no PD is arranged can bestarted in sync with the false sync signal which is delayed by apredetermined time from the first sync signal obtained from the PD 110.

FIG. 31 is a timing chart for operation of the false sync signalgenerating part in the second embodiment. In the example of FIG. 31,three counters (counters 0 to 2) are used to illustrate the case wherethe two photoconductors are scanned by the two of the faces in thepolygon mirror.

As shown in FIG. 31, the first sync signal is asserted in sync with thescanning of the photoconductor drum 103 at the time of the scan start ofthe face-4 of the polygon mirror. The counters 0-2 are reset one by oneby the first sync signal for the three faces, and the counting isstarted by the first pixel clock.

If the counter 0 is reset for the face-1, the scan start timing of thephotoconductor drum 104 using the face-1 is the timing set up by thefalse sync count number Nref_ps of the comparator 1 after one or more ofthe faces of the polygon mirror are rotated. The timing of the falsesync signal is generated so that the light source 127 writes on thephotoconductor drum 104 for the same face of the polygon mirror as theface of the polygon mirror used when the light source 117 writes on thephotoconductor drum 103.

As described above, in the second embodiment, the frequency of the firstpixel clock is controlled according to the rotational speed irregularityof the polygon mirror, a false sync signal is generated based on thefirst pixel clock with the controlled frequency, and the start positionof the light beam on the opposite side is fixed. The error for the facesof the polygon mirror can be eliminated by generating the false syncsignal. By setting up the initial frequency for each pixel clockgenerating part, the error of each scanning optical system can becorrected, and the error of the write end position can be corrected.

The image modulation data is synchronized with the second pixel clockbased on the image data, and the image modulation data is output as thelight beam in the laser drive circuit.

Next, a third embodiment of the present disclosure will be described.FIG. 32 is a diagram showing the composition of the image forming device10 of the third embodiment of the present disclosure.

As shown in FIG. 32, the image forming device 10 of the third embodimentincludes one light source, two photoconductor drums, and one syncdetecting sensor (PD 110).

Apart from the second embodiment, in the image forming device 10 of thethird embodiment, the laser beam emitted from the LD 117 is deflected byan optical beam division unit 107 to generate incident light Y andincident light Bk. The function of the pixel clock generating part-2(142) to generate the second pixel clock 124 and the function of thepixel clock generating part-3 (144) to generate the third pixel clock129 as shown in FIG. 30 are performed in this embodiment as follows. Thepixel clock generating part-4 (153) in the third embodiment of FIG. 32changes the frequency of the pixel clock 4 for one of the period thesurface of the photoconductor drum 103 is scanned and the period thesurface of the photoconductor drum 104 is scanned. The fifth modulationdata generating part 129 in the third embodiment of FIG. 32 is provided,instead of the first data modulation part 145 and the second modulationdata generating part 147 in the embodiment of FIG. 30. The fifth laserdriver (155) in the third embodiment of FIG. 32 is provided, instead ofthe first laser driver (146) and the second laser driver (148) in theembodiment of FIG. 30.

In the third embodiment of FIG. 32, the image forming device 10 includesa polygon mirror 100, two scanning lenses 101 and 102, twophotoconductor drums 103 and 104, two incidence mirrors 105 and 106, anoptical beam division unit 107, a PD 110, a pixel clock generatingdevice 150, a modulation data generating part-5 (129), a laser driver 5(155), and a light source (LD) 117.

The pixel clock generating device 150 includes a pixel clock generatingpart-1 (151), a false sync signal generating part 152, and a pixel clockgenerating part-4 (153). The laser beam from the light source 117 isdivided into incident light Bk (the first light beam) and incident lightY (the second light beam) by the optical beam division unit 107.

The incident light Bk and the incident light Y enter the face where theywere reflected by the incidence mirrors 105 and 106, surfaces of thepolygon mirrors 100, passing through the scanning lenses 101 and 102,and scanned onto the photoconductor (Bk) 103 and the photoconductor (Y)104, respectively. Thereby, the electrostatic latent images according tothe output of the light source 117 are formed on the photoconductordrums 103 and 104, respectively.

The PD 110 is arranged at an end portion of the photoconductor drum 103.The laser beam reflected by the polygon mirror 100 enters the PD 110,before carrying out one-line scanning of the photoconductor drum 103,and the timing of a start of the scanning is detected by the PD 110.

The timing of the start of the scanning detected by the PD 110 issupplied to the pixel clock generating part-1 (151), false sync signalgenerating part 152, and pixel clock generating part-4 (153) of pixelclock generating device 150 as the first periodic sync signal unitedwith the scanning of the photoconductor.

The pixel clock generating part-1 (151) generates pixel clock 1 and afrequency correction value based on the first sync signal. The falsesync signal generating part 152 generates a false sync signal based onthe first sync signal and the pixel clock 1.

The pixel clock generating part-4 (153) generates the pixel clock 4 bythe frequency correction value of the first sync signal and the falsesync signal. Based on the image data, the modulation data generatingpart-5 (129) generates the modulation data in sync with pixel clock 4and outputs the same to the laser driver 5 (155), so that the lightsource 117 is driven according to the modulation data of the laserdriver 5 (155) to output a laser beam.

The scanning optical system of FIG. 32 is arranged to divide the laserbeam into incident light Bk and incident light Y using the light beamdivision unit 107 which uses a half mirror.

FIG. 33 is a block diagram showing the composition of the pixel clockgenerating part-4 (153). As shown in FIG. 33, the pixel clock generatingpart-4 (153) includes an initial frequency setting unit 601 which setsup the initial frequency of pixel clock 4 to scan the photoconductordrum 103, an initial frequency setting unit 602 which sets up theinitial frequency of pixel clock 4 to scan the photoconductor drum 104,an adder 603, an OR circuit 605, a digital control oscillator (DCO) 604,a selector 606, and a side signal generating part 607.

The side signal generating part 607 detects which of the photoconductordrums 103 and 104 is scanned, and changes a side signal to H or Laccording to the first sync signal and the false sync signal. Theinitial frequency setting unit 601 holds the first initial frequencysetting value, and the initial frequency setting unit 602 holds thesecond initial frequency setting value.

The adder 603 adds the frequency correction value generated by the firstpixel clock generating part 112 and one of the initial frequency settingvalues selected by the selector 606. 5 In the phase sync with the syncsignal, the digital control oscillator (DCO) 604 generates the pixelclock 4.

FIG. 34 is a timing chart for explaining operation of the pixel clockgenerating part-4 (153).

As shown in FIG. 34, the first sync signal is asserted when the incidentlight Bk is detected by the PD 110. The false sync signal is asserted inthe above-mentioned timing after the first sync signal is asserted. Theside signal is changed to a high (H) level if the false sync signal isasserted, and changed to a low (K) level if the first sync signal isasserted. The sync signal is the sum of the first sync signal and thefalse sync signal. The first initial frequency setting value and thesecond initial frequency setting value are stored in the register.

Ma denotes one of the setting value ma of the first initial frequencyand the setting value mb of the second initial frequency which isselected by the side signal. When the side signal is at a low (L) level,ma of the first initial frequency is selected, and mb of the secondinitial frequency is selected when the side signal is at a high (H)level. Mi denotes a frequency setting value of the pixel clock which isdefined by the formula: Mi=(Ma+frequency correction value Δf). Hence,the frequency setting value Mi of the pixel clock is set to Mi=ma+Δf orMi=mb+Δf in accordance with the side signal. The frequency of the pixelclock is updated to Mi=ma+Δf or Mi=mb+Δf by the sync signal, and in thephase in sync with the sync signal, the digital control oscillator 604outputs the pixel clock 4.

Because the pixel clock generating part-4 (153) and the pixel clockgenerating part-1 (151) can be set up to different initial frequencyvalues, the setting of the initial frequency value can be performed toeliminate the error of each scanning optical system. According to thethird embodiment, the initial value of frequency can be set up toeliminate the error of each scanning optical system, and it is possibleto correct the error of the write end positions.

As mentioned above, in the third embodiment, the write start position ofthe light beam on the opposite side is fixed by controlling thefrequency of the pixel clock 1 according to the rotational speedirregularity of the polygon mirror, and the false sync signal isgenerated based on the pixel clock 1 with the controlled frequency. Theerror of each face of the polygon mirror can be disregarded bygenerating the false sync signal for the face of the polygon mirrorwhich is the same as that of the first sync signal. By setting up pluralinitial frequencies individually with a single LD, the error for eachscanning optical system can be corrected, and the error of the write endposition can be corrected.

Next, a fourth embodiment of the present disclosure will be described.FIG. 35 is a diagram showing the composition of the image forming device10 of the fourth embodiment of the present disclosure.

As shown in FIG. 35, the image forming device 10 of the fourthembodiment includes four write laser diodes, four photoconductor drums,and two sync detecting sensors.

When compared with the image forming device 10 of the second embodimentshown in FIG. 30, the image forming device 10 shown in FIG. 35 furtherincludes photoconductor drums 703 and 704, incidence mirrors 705 and706, a PD 707, a third laser driver 1915, a fourth laser driver 1909,and light sources 713 and 1901 which are extended as the optical system.The pixel clock generating device 1900 further includes a second falsesync signal generating part 1902, a clock generating part-5 (1910), athird modulation data generating part 1913, a pixel clock generatingpart-6 (1904), and a fourth modulation data generating part 1907.

The incident light M and the incident light Bk pass through the scanninglens 101 in common, and the incident light Y and the incident light Cpass through the scanning lens 102 in common.

The pixel clock generating part-2 (142), the pixel clock generatingpart-3 (144), the first modulation data generating part 145, the secondmodulation data generating part 147, the first laser driver 146, and thesecond laser driver 148 in the fourth embodiment are essentially thesame as the pixel clock 2 (142), the pixel clock generating part-3(144), the first modulation data generating part 145, the secondmodulation data generating part 147, the first laser driver 146, and thesecond laser driver 148 in the second embodiment which have beendescribed above with reference to FIG. 30, and a description thereofwill be omitted.

The pixel clock generating part-5 (1910) and the sixth pixel clockgenerating part (1904) in the fourth embodiment are essentially the sameas the pixel clock 2 (142) and the pixel clock generating part-3 (144)in the second embodiment, and a description thereof will be omitted. Thethird modulation data generating part 1913 and the fourth modulationdata generating part 1907 in the fourth embodiment are essentially thesame as the first modulation data generating part 145 and the secondmodulation data generating part 147 in the second embodiment, and adescription thereof will be omitted. Further, the third laser driver1915 and the fourth laser driver 1909 in the fourth embodiment areessentially the same composition as the first laser driver 146 and thesecond laser driver 148 in the second embodiment, and a descriptionthereof will be omitted.

FIG. 36 is a timing chart for explaining operation of the false syncsignal generating part in the fourth embodiment.

When compared with the timing chart in the second embodiment in FIG. 31,in the operation of the false sync signal generating part shown in FIG.36, with respect to the LD 117 and the LD 713, the first sync signalfrom the PD 110 and the second sync signal from the PD 707 are input tocause the comparators a0-a2 and b0-b2 and the counters a0-a2 and b0-b2to operate so that the first false sync signal and the second false syncsignal are generated, respectively. Other operations of the false syncsignal generating part in the fourth embodiment are essentially the sameas in the timing chart of FIG. 31, and a description thereof will beomitted.

In the timing chart of FIG. 36, the timing when the false sync signal isgenerated is the same as the timing the surface of the photoconductordrum 104 is scanned by the light beam from the same face of the polygonmirror 100 when the light beam from the LD 117 scans the surface of thephotoconductor drum 103. The timing when the second false sync signal isgenerated is the same as the timing the surface of the photoconductordrum 704 is scanned by the light beam from the same face of the polygonmirror 100 when the light beam from the LD 713 scans the surface of thephotoconductor drum 703.

In the fourth embodiment, the frequency of the first pixel clock iscontrolled according to the rotational speed irregularity of the polygonmirror, and a false sync signal is generated based on the controlledfrequency of the first pixel clock, so that the write start position ofthe light beam for the opposite side of the opposed scanning sides isfixed.

In the fourth embodiment, the first false sync signal is generated forthe same face as that of the first sync signal, and the errors for therespective faces of the polygon mirror can be disregarded. Similarly,the second false sync signal is generated for the same face as that ofthe second sync signal, and the errors for the respective faces of thepolygon mirror can be disregarded. The setting of the initial frequencyis performed for each LD, and the error for each scanning optical systemcan be corrected. Even if the number of photoconductor drums isincreased to be larger than that in the second embodiment, the error ofthe write end position can be corrected.

Next, a timing chart in which a sync signal detected by the PD 707 isdivided into a first sync signal and a second sync signal will bedescribed with reference to FIG. 37. FIG. 37 is a timing chart in whichthe first sync signal and the second sync signal are separated from thesync signal.

As shown in FIG. 37, the sync signal detected by the PD 707 is allocatedto either the first sync signal or the second sync signal in accordancewith a sync select signal obtained from the CPU or the control unit ofthe host device. In the example of FIG. 37, when the sync select signalis at a low level L, the sync signal is separated to the first syncsignal, and when the sync select signal is at a high level H, the syncsignal is separated to the second sync signal.

With the use of the sync select signal obtained from the control unit ofthe host device or the CPU, the sync signal can be separated into thefirst sync signal and the second sync signal even if a single PD is usedin common.

Next, a fifth embodiment of the present disclosure will be described.FIG. 38 is a diagram showing of the composition of the image formingdevice 10 of the fifth embodiment of the present disclosure.

As shown in FIG. 38, the image forming device 10 of the fifth embodimentincludes two light sources, four photoconductor drums, and two syncdetecting sensors.

When compared with the image forming device of the third embodiment inFIG. 32, the image forming device 10 shown in FIG. 38 further includesphotoconductor drums 703 and 704, incidence mirrors 705 and 706, a PD707, a laser driver 712, a light source 713, and a modulation datagenerating part 711 that outputs the second modulation data to the laserdriver 712.

The LD 713 which is disposed at a position distant from the LD 117 isillustrated in FIG. 38. However, in the actual optical system, the LD713 may be disposed at the position distant from the LD 117 in thesub-scanning direction. The incident light M and the incident light Bkpass through the scanning lens 101 in common, and the incident light Yand the incident light C pass through the scanning lens 102 in common.

In the fifth embodiment, the same operation as the third embodiment isperformed. However, in the fifth embodiment, the setting of an initialfrequency is performed for each of the pixel clock generating part-4(128) and the pixel clock generating part-7 (710), and the error foreach scanning optical system can be corrected.

The pixel clock generating part-4 (128) and the pixel clock generatingpart-7 (710) have the same composition as the pixel clock generatingpart-4 (131) shown in FIG. 33, and a description thereof will beomitted. The pixel clock generating part-4 (128) generates a pixel clock4 which is a reference clock of the incident light Bk and the incidentlight Y, and the pixel clock generating part-7 (710) generates a seventhpixel clock which is a reference clock of the incident light M and theincident light C.

The polygon mirror 100 can be shared by the pixel clock generatingpart-4 (128) and the pixel clock generating part-7 (710), and rotationirregularity of each pixel clock generating part is equal. Hence, thepixel clock generating part-1 (112) can be used with these pixel clockgenerating parts, and it is possible to provide a simple structure ofthe circuit.

The operation of the false sync signal generating part in the fifthembodiment is essentially the same as the operation in the timing chartof FIG. 36 regarding the fourth embodiment, and a description thereofwill be omitted.

Next, a sixth embodiment of the present disclosure will be described.FIG. 39 is a diagram showing the composition of an image forming deviceof the sixth embodiment of the present disclosure.

In the image forming device in FIG. 39, the PD 110 in the image formingdevice of the fifth embodiment shown in FIG. 38 is omitted, only the PD707 remains, and a selector 1001 is installed.

As shown in FIG. 39, the first sync signal from the PD 707 is shared bythe false sync signal generating part 709 and the pixel clock generatingpart-7 (710). Although the single PD 707 is used, the write timing ofthe LD 117 and the write timing of the LD 713 differ from each other,and the false sync signal generating part uses the false sync signalgenerating part 709 and the second false sync signal generating part1902, similar to the fifth embodiment.

The LD 713 which is disposed at a position distant from the LD 117 isillustrated in FIG. 39. However, in the actual optical system, the LD713 may be disposed in the vicinity of the LD 117.

In the sixth embodiment, the same operation as the fifth embodiment isperformed. However, the setting of an initial frequency is performed foreach of the pixel clock generating part-4 (128) and the pixel clockgenerating part-7 (710), and the error for each scanning optical systemcan be corrected.

The pixel clock generating part-4 (128) and the pixel clock generatingpart-7 (710) have the same composition as the pixel clock generatingpart-4 (131) shown in FIG. 33, and a description thereof will beomitted. The pixel clock generating part-4 (128) generates a pixel clock4 which is a reference clock of the incident light Bk and the incidentlight Y. The pixel clock generating part-7 (710) generates a seventhpixel clock which is a reference clock of the incident light M and theincident light C. The polygon mirror 100 is shared by the pixel clockgenerating part-4 (128) and the pixel clock generating part-7 (710), androtation irregularities of the pixel clock generating parts are equal.Hence, the first pixel clock generating part 112 can be used with thesepixel clock generating parts, and it is possible to provide a simplestructure of the circuit.

FIG. 40 is a timing chart for explaining operation of the false syncsignal generating part in the sixth embodiment. The operation of thefalse sync signal generating part shown in FIG. 40 is essentially thesame as shown in FIG. 36, and the first sync signal from the PD 110 isused to cause the comparators a0-a2 and b0-b2 and the counters a0-a2 andb0-b2 to operate so that the first false signal and the second falsesignal are generated.

According to the optical scanning apparatus of the present disclosure,even when a sync signal generating unit (PD) is arranged only on oneside of the opposed scanning sides, based on the sync signal of the syncsignal generating unit, a false sync signal for the opposite side of theopposed scanning sides is generated with high precision. Therefore, theabove-mentioned errors are corrected and the write start position andthe write end position can be corrected with high precision.

The present disclosure is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present disclosure.

The present application is based upon and claims the benefit of priorityof Japanese Patent Application No. 2011-277161, filed on Dec. 19, 2011,Japanese Patent Application No. 2012-023572, filed on Feb. 7, 2012, andJapanese Patent Application No. 2012-265706, filed on Dec. 4, 2012, thecontents of which are incorporated herein by reference in theirentirety.

What is claimed is:
 1. An image forming device including a processingunit to control the image forming device, the processing unitcomprising: a first sync signal generating part configured to detectscanning timing of a first photoconductor and generate a first syncsignal; a first counter configured to receive the first sync signal anda first pixel clock and measure a rotational period of a rotary polygonmirror; a first pixel clock generating part configured to adjust thefrequency of the first pixel clock to make the rotational periodmeasured by the first counter equal to a predetermined value; and afalse sync signal generating part configured to generate a false syncsignal for measuring timing of scanning of a second photoconductor,based on the first sync signal and the first pixel clock.
 2. The imageforming device according to claim 1, wherein the first pixel clockgenerating part outputs the first pixel clock and a frequency correctionvalue which is a difference between a frequency of the first pixel clockand a predetermined frequency, and wherein the processing unit furthercomprises: a second pixel clock generating part configured to output asecond pixel clock that is in sync with the first sync signal, at afrequency corrected by the frequency correction value; and a third pixelclock generating part configured to output a third pixel clock that isin sync with the false sync signal, at the frequency corrected by thefrequency correction value.
 3. The image forming device according toclaim 1, wherein the first counter is configured to measure an inputinterval of the first sync signal for each of the faces of the polygonmirror and output an average of the intervals of the first sync signalmeasured for the number of the faces of the polygon mirror.
 4. The imageforming device according to claim 2, wherein an initial value of thefrequency of the second pixel clock and an initial value of thefrequency of the third pixel clock are predetermined individually. 5.The image forming device according to claim 4, wherein the processingunit further comprises: a fourth pixel clock generating part configuredto generate a fourth pixel clock which is oscillated in sync with thefirst sync signal at a frequency obtained from a first initial frequencysetting value corrected by the frequency correction value, or oscillatedin sync with the false sync signal at a frequency obtained from a secondinitial frequency setting value corrected by the frequency correctionvalue.
 6. The image forming device according to claim 1, wherein theprocessing unit further comprises: a second sync signal generating partconfigured to detect a timing of scanning of a third photoconductor by athird light beam deflected by the polygon mirror and generate a secondsync signal; and a second false sync signal generating part configuredto generate a second false sync signal which measures timing of scanningof a fourth photoconductor by a fourth light beam deflected by thepolygon mirror, using the first pixel clock.
 7. The image forming deviceaccording to claim 6, further comprising: a fifth pixel clock generatingpart configured to output a fifth pixel clock that is in sync with thesecond sync signal at a frequency corrected by the frequency correctionvalue; and a sixth pixel clock generating part configured to output asixth pixel clock that is in sync with the second false sync signal atthe frequency corrected by the frequency correction value.
 8. The imageforming device according to claim 7, wherein an initial value of thefrequency of the second pixel clock, an initial value of the frequencyof the third pixel clock, an initial value of the frequency of the fifthpixel clock, and an initial value of the frequency of the sixth pixelclock are predetermined individually.
 9. The image forming deviceaccording to claim 8, wherein the processing unit further comprises: aseventh pixel clock generating part configured generate a seventh pixelclock which is oscillated in sync with the second sync signal at afrequency obtained from a third initial frequency setting valuecorrected by the frequency correction value, or oscillated in sync withthe second false sync signal at a frequency obtained from a fourthinitial frequency setting value corrected by the frequency correctionvalue.